1. Field of the Invention
The present invention generally relates to radiation detectors and image intensifiers and, more particularly, to composite photocathodes in radiation detectors and image intensifiers for directly converting x-ray and gamma-ray radiation imagery into electron imagery.
2. Background Description
X-ray to visible light converters are well known in the art, but generally use indirect conversion techniques where x-ray energy is converted to visible light in a scintillator and the visible light (photons) is then converted to a corresponding electron image. In the instance of an image intensifier, the secondary electrons can be multiplied in a microchannel plate or accelerated by an electric field before striking a phosphor display screen to provide an enhanced directly viewable visible image. There are numerous disadvantages in having to convert an x-ray image to a visible light image before generating and multiplying a corresponding electron image. Conversion of an x-ray image to a visible light image is normally accomplished by using a scintillator, as is well known in the art and as described in U.S. Pat. No. 4,104,516, 4,140,900, 4,255,666 and 4,300,046. In each instance, the scintillator exhibits a limited response time, poor spatial resolution, and due to the complicated fabrication techniques and the attendant requirement to use light shielding, it becomes extremely expensive to fabricate.
The x-ray to electron converter referenced above will not function with gamma-ray radiation. Gamma-ray imaging systems utilize very sophisticated techniques which contain a plurality of scintillators and photomultipliers; therefore, the imaging system becomes unduly complicated and cost prohibitive. One of the primary reasons that the development of gamma-ray image intensifiers have not paralleled the x-ray image intensifiers is that the gamma-rays have much higher photon energy than x-rays and, in the case of indirect conversion techniques as used in x-ray image intensifiers, the scintillator must be thicker for gamma rays than that of an x-ray detector, and under these conditions the light spreads in the thicker scintillator and the spatial resolution becomes severely degraded, rendering it impossible to provide useful spatial information. There are two counter productive principles existing in the fabrication of prior imaging detectors for high energy photons; i.e., the thicker the scintillator, the higher the quantum detection efficiency, but the worse the spatial resolution becomes.
While the prior art has reported various types of x-ray/gamma-ray to electron converters, none have dealt specifically with resolving the specific problem of providing a high efficiency, direct conversion x-ray or gamma-ray photocathode.